Bioaerosols are relevant for public health and may play
an important role in the climate system, but their atmospheric abundance,
properties, and sources are not well understood. Here we show that the
concentration of airborne biological particles in a North American forest
ecosystem increases significantly during rain and that bioparticles are
closely correlated with atmospheric ice nuclei (IN). The greatest increase
of bioparticles and IN occurred in the size range of 2–6 μm, which is
characteristic for bacterial aggregates and fungal spores. By DNA analysis
we found high diversities of airborne bacteria and fungi, including groups
containing human and plant pathogens (mildew, smut and rust fungi, molds,
<i>Enterobacteriaceae, Pseudomonadaceae</i>). In addition to detecting known bacterial and fungal IN (<i>Pseudomonas</i> sp., <i>Fusarium sporotrichioides</i>), we
discovered two species of IN-active fungi that were not previously known as
biological ice nucleators (<i>Isaria farinosa</i> and <i>Acremonium implicatum</i>). Our findings suggest that atmospheric
bioaerosols, IN, and rainfall are more tightly coupled than previously
assumed.